The consumption of seafood species for which there is a high consumer demand such as salmon, trout, halibut and eel is increasing and due to this high demand and limited natural stocks, much effort is spent on developing cost effective aquacultural methods of farming such species. A particularly serious problem is to secure a high survival rate of the hatched larvae of the species being cultivated.
Expansion of the aquaculture industry requires that several problems be addressed, one of the most significant being the difficulty of supplying live prey organisms which provide a nutritionally adequate feed for the larvae. Larval fish in the wild consume a mixed population of phytoplankton prey organisms that provide a balanced nutrition. However, collecting phytoplankton in sufficient quantities to meet the demand in aquaculture is not feasible. As an alternative, selected species of prey organisms, in particular rotifers and Artemia species, are presently cultivated and used as feed.
Generally however, such artificially cultivated prey organisms, although they provide adequate amounts of protein and energy, have a lipid composition which is not adequate to cover the requirement for certain HUFAs, in particular DHA and EPA which are essential for the optimum survival, growth and development of larvae. Specifically, it has been shown that a high content of DHA is required and that the ratio between DHA and EPA in the prey organisms should be at least 1:1 and preferably at least 2:1. To provide prey organisms having such a composition in respect of HUFAs it is necessary to cultivate the organisms in the presence of enrichment compositions having a high content of DHA, preferably at least 20 wt % and a ratio of DHA to EPA exceeding the ratio aimed at in the prey organisms, such as at least 3:1 and preferably higher.
Currently, this problem is being addressed by cultivating the prey organisms in the presence of enrichment compositions permitting the organisms to be enriched in respect of these essential fatty acids. However, presently available commercial compositions for that purpose such as emulsion products sold under the tradename Selco™, do not meet the above requirements in that the DHA content is relatively low and/or the DHA:EPA ratio is not high enough. Using Artemia enriched with these commercial compositions survival rates of fish larvae in the range of 12 to 15% have been reported (McEvoy et al. Aquaculture 163 (1998) 237–250; Navarro et al. J. Fish Biol. 43 (1993) 503–515). In this context, survival rates are defined as survival percentage from the first feeding through metamorphosis. For cost-effective aquaculture production a larval survival rate of 50% and preferably higher should be obtained.
Other commercially available compositions for prey organism enrichment are products sold under the tradename Algamac™ containing up to 14 wt % of DHA, and tuna orbital oil (TOO) that contains up to 30 wt % of DHA.
WO 99/37166 discloses a method for the enrichment of live prey organisms with nutrients essential for fish larvae based on the use of dry fatty acid soap powders of HUFAs obtained from the waste stream of marine algae oil extraction. The raw material for providing these powders has a content of phospholipids and about 28 wt % of free fatty acids and it contains about 23 wt % of DHA but apparently no other n-3 fatty acids. Artemia enrichment levels of about 2.7% DHA of dry weight are disclosed.
Another material intended for use in aquaculture is described in WO 99/06585. Examples disclose a DHA content of 24 wt %, but the phospholipid content is not disclosed. The material however, contains a high proportion of free fatty acids (about 32–37 wt %) and a high content of non-lipid material (about 39–44 wt %), which may reduce the lipid uptake efficiency of prey animals. A high content of free fatty acids is generally considered harmful for fish larvae and juveniles.
Neither of the two last-mentioned materials is fish-based and they lack many HUFAs found in fish, such as EPA and other n-3 fatty acids.
In a recent review by Sargent et al. (Aquaculture 179 (1999) 217–229) it is emphasized that in addition to the requirement in respect of HUFAs, fish larvae have a dietary requirement for phospholipids and it is stressed that the ideal diet for fish larvae is a diet having a composition similar to the yolk of the eggs. According to these authors fish egg yolk contains about 10 wt % (on a dry matter basis) phospholipids which contain about 17 wt % of DHA an about 9 wt % of EPA. These authors conclude in their review that a problem remains with respect to how to construct such a diet on a commercial scale from currently available materials.
To our knowledge, it has not been possible, with the use of the above-mentioned commercial feed compositions, or other prior art experimental compositions, to obtain DHA enrichment levels in Artemia that approach an ideal, egg yolk-similar diet.
It has now been found that it is possible to provide a lipid composition for enriching aquacultural prey organisms based on the use of DHA-rich phospholipids isolated from abundantly available marine organism materials such as fish meal. By using this starting material it has become possible to provide enrichment compositions on a commercial scale, which make it possible to provide prey organisms having, in respect of HUFAs and phospholipids, a composition which is very close to that of fish egg yolk and which are therefore highly appropriate to secure optimum survival, growth, pigmentation and morphogenesis of fish larvae such as halibut larvae.